This invention relates to a method and apparatus for reducing drop placement error of ink drops emitted by printheads, e.g. acoustic ink printing (AIP) printheads, in printers. Ink types include aqueous and phase change (hot melt), with finite electrical conductivity to allow inductive charging at drop emission. More particularly, two schemes are contemplated to facilitate reduction of drop placement error, preferably to zero, for printing on print medium. In both schemes, segmented counter electrodes are biased at iteratively predetermined voltages and located across a print gap from drop ejectors integrated in the printheads. For printing on stationary medium such as paper on a belt or platen, absolute drop placement error for each ejector row is maintained to be zero to obtain the required bias electrode voltages. For printing on moving medium such as paper on a drum, the same time of flight for all ejector rows referenced to the 1st row is maintained, thus resulting in the relative drop placement error being zero and the absolute error being negligibly small.
While the invention is particularly directed to the art of drop placement in the context of acoustic ink printing where the print medium is or is disposed on a curved surface, and will be thus described with specific reference thereto, it will be appreciated that the invention may have usefulness in other fields and applications. For example, the invention may be used to print on planar surfaces as well as in a variety of ink jet printing applications.
By way of background and generally, in many printing applications, drops of ink are ejected or emitted on demand and deposited onto a medium to form the printed image. For high resolution print images, a combination of small drop size and precise drop placement are necessary to ensure good image quality. The requirement for accurate drop placement is especially critical for color printing on moving non-planar print media such as drums or belts.
More specifically, acoustic ink printing involves the emission of a droplet of ink from a pool of ink toward a print medium. Acoustic waves are generated and focussed toward the surface of the ink pool to emit the droplet therefrom. While acoustic ink printing elements may take various forms, such elements typically include a piezoelectric transducer to generate the acoustic waves, a lens to focus the waves at the surface of the ink pool, a cover plate with apertures formed therein to allow emission of the ink, and corresponding wiring. It is to be appreciated that approximately one thousand (1,000) or more of these elements may be disposed on a single printhead in a variety of configurations. Typically, however, the printing elements are formed in eight rows along the length of the printhead. Acoustic ink printing systems are disclosed, for example, in U.S. Pat. Nos. 4,308,547; 4,697,195; 5,028,937; and 5,087,931, all of which are incorporated herein by reference.
The advantages of electrostatic field acceleration in reducing drop placement errors for both aqueous and phase change AIP efforts are known. Errors in drop placement due to transverse disturbances, such as airflow and skewed drop ejection, are reduced by providing a Coulomb force component normal to the printing surface to attract the drops. This force also acts to overcome drag, thus providing an impetus for the drop to move across the print gap. Otherwise, the drop may decelerate and fall back onto the print head leading to contamination problems that adversely affect print head reliability and lifetime. Another important advantage is the reduction in mechanical energy for drop ejection by supplying just sufficient energy for drop formation; and then using the electrostatic field to accelerate the drop. This measure results in significant reduction of power.
U.S. Pat. Nos. 4,386,358 and 4,379,301 to Fischbeck, which are commonly assigned and incorporated herein by reference, disclose a method for electrostatically deflecting electrically charged ink drops emitted from an ink jet printhead. Charges placed on electrodes on the printhead disclosed by Fischbeck are controlled to steer the charged ink drops in desired directions to compensate for known printhead movement. By electrostatically steering the charged ink drops, the method disclosed in Fischbeck compensates for ink drop misdirection caused by the known printhead movement when the ink drop is emitted.
However, the electrostatic deflection method disclosed by Fischbeck does not compensate for unpredictable environmental factors that can affect ink drop trajectories. Such environmental factors include air currents and temperature gradients between the printhead and the print substrate. In acoustic ink jet printheads, unpredictable variations in the dynamics of ink drop creation also detrimentally affect ink drop trajectories. Some of the variations in ink drop creation are caused by aberrations in the lithography of Fresnel lens which are in some embodiments used to focus the acoustic wave used to create the ink drops.
U.S. Pat. No. 5,975,683 entitled Electric-Field Manipulation of Emitted Ink Drops in Printing, which is commonly assigned, and is hereby incorporated by reference, discloses the use of an electric field to reduce droplet misdirectionality, by inducing a charge on a drop as it breaks off from the bulk of the fluid. The charged drop is then accelerated into the paper, by holding the paper at a relatively large potential (this same potential may be used to induce the charge on the drop). The application teaches selectively deflecting the ink drops slightly to enhance the resolution of the image produced by a given printhead configuration. The ink jet actuators form and impart an initial velocity on the ink drops. The charged ink drops are then steered by electrodes such that the drops alternately impact upon the print medium at positions slightly offset from positions directly opposite the apertures of the printhead.
This approach, though useful, has drawbacks. It requires large voltages, of the order of 1 to 2 kV. Also, it will suffer from many of the same imaging artifacts as occur in ionographic printing, where because charge is deposited onto the printing substrate, there is print-dependent interaction of the accelerating field with the charged drop. That is, as drops are accumulated on the paper, so is their charge. If this charge is not removed quickly enough, it will produce a print-dependent potential at the paper surface, which will interfere with the acceleration of subsequent drops. Finally, the acceleration expected for drops under typical print conditions is only large enough to reduce the misplacement of drops by some 50% at the paper surface, so that the correction of the misdirection, while significant, is not complete.
U.S. patent application Ser. No. 08/721,290 (filed Sep. 26, 1996) entitled Method and Apparatus for Moving Ink Drops Using an Electric Field, which is commonly assigned, and is hereby incorporated by reference, discloses using an electric field to charge and impart a force onto ink drops to control for motion of the ink drops, including biasing the print support medium with a voltage source.
U.S. patent application Ser. No. 09/098,763 (filed Jun. 17, 1998) entitled xe2x80x9cReduction of Spot Misplacement Through Electrostatic Focusing of Uncharged Dropsxe2x80x9d, which is commonly assigned and hereby incorporated herein by reference, is directed to lateral focus of aqueous ink drops onto a substrate through implementation of electric fields for use in acoustic ink printing.
Known techniques do not take into account, however, that the print medium may be non-planar, e.g. comprised of a curved surface. These techniques only effectively contemplate the placement of drops on a planar medium. This is significant because the geometry of the print medium presents an additional complicating source for drop placement error. Addressing the problems associated with printing on a curved surface is particularly important in high volume printing systems where drums are used in the system to increase productivity.
The present invention contemplates a new and improved apparatus and method useful for realizing reduced, e.g. zero, drop placement error in printing applications, e.g. acoustic ink printing applications, that resolve the above-referenced difficulties and others.
A method and apparatus for reducing drop placement error in printing systems are provided. The printing systems have a printhead positioned to emit drops of ink toward target positions on a print medium positioned on a curved surface. The printhead has rows of emitters and the curved surface has embedded therein segmented electrodes. The electrodes are respectively aligned with the rows.
In one aspect of the invention, the method comprises steps of iteratively determining voltages to apply to the electrodes, biasing the electrodes based on the determined voltages, and, selectively emitting the drops of ink from emitters such that the drops follow respective paths from the emitters to the target positions on the print medium based on the biasing and position of the electrodes relative to the print medium.
In another aspect of the invention, the determining of the voltages is based on whether the print medium is in motion during the emitting.
In another aspect of the invention, if the print medium is in motion during the emitting, the voltages are determined based on maintaining a substantially identical time of flight for the emitted drops.
In another aspect of the invention, if the print medium is stationary during the emitting, the voltages are determined to achieve substantially zero absolute error for drop placement.
In another aspect of the invention, the apparatus comprises a head having rows of fluid emitters disposed thereonxe2x80x94the emitters including apertures formed in a cover plate of the printhead and the cover plate being connected to ground, a controller operative to control emission of drops of fluid from the emitters, a curved surface having embedded therein electrodes aligned with the rows of emittersxe2x80x94the curved surface being positioned across a gap from the head, and a processor operative to iteratively determine respective voltages to bias the electrodesxe2x80x94wherein the drops of fluid are selectively emitted from the emitters of the printhead based on signals from the controller and emitted such that the drops follow respective paths from the grounded cover plate of the emitters to the target positions on the print medium based on the biasing and position of the electrodes relative to the print medium.
In another aspect of the invention, the head is stationary.
In another aspect of the invention, the apparatus further comprises means for moving the head relative to the print medium during printing.
In another aspect of the invention, the curved surface is disposed on a drum.
In another aspect of the invention, the curved surface is disposed on a shoe.
In another aspect of the invention, the apparatus further comprises means for moving the print medium relative to the head.
In another aspect of the invention, the processor includes means for determining the voltages based on whether the print medium is in motion during the printing.
In another aspect of the invention, if the print medium is in motion during the emitting, the determining means determines the voltages based on criteria to maintain a substantially identical time of flight for the emitted drops.
In another aspect of the invention, if the print medium is stationary during the emitting, the determining means determines the voltages to achieve substantially zero absolute error for drop placement.
In another aspect of the invention, the apparatus comprises means for emitting drops of ink toward target positions on a print mediumxe2x80x94the emitting means having rows of emitters, means for supporting the print mediumxe2x80x94the supporting means having embedded therein segmented electrodes and the electrodes being respectively aligned with the rows, means for iteratively determining voltages to apply to the electrodes, means for biasing the electrodes based on the determined voltages, and means for selectively emitting the drops of ink from the emitting means such that the drops follow respective paths to the target positions on the print medium based on the biasing and position of the electrodes relative to the print medium.
In another aspect of the invention, the means for determining the voltages bases the determination on whether the print medium is in motion during the emitting.
In another aspect of the invention, if the print medium is in motion during the emitting, the voltages are determined based on maintaining a substantially identical time of flight for the emitted drops.
In another aspect of the invention, if the print medium is stationary during the emitting, the voltages are determined to achieve substantially zero absolute error for drop placement.
Further scope of the applicability of the present invention will become apparent from the detailed description provided below. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art.